Tweak some factories to support stubbing

What does this MR do?

This was extracted from !28195 (merged), and allows us to use build_stubbed with more models/factories.

• Use association() instead of create() in association blocks, to avoid enforcing the builder strategy.
• Pass on project in association blocks instead of after callbacks.
• Add has_internal_id trait to generate IIDs on stubbed models.
• Add ProjectWiki#id accessor.
• Rename discussion_note_on_snippet to *_project_snippet, for consistency with the note_* factories.
• Add #sha on stubbed commits in RepoHelpers.

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers
• [-] Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• [-] Label as security and @ mention @gitlab-com/gl-security/appsec
• [-] The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• [-] Security reports checked/validated by a reviewer from the AppSec team